I've written about Temporary Dimensions and the Activate Dimension button in the past (2007-9), these are some posts that discuss them.
Space Bar Subtle Effect on Temporary Dimensions
Dept. of Reviteristics - Activate Dimensions
Activate Dimensions - Redux
Temporary Dimensions don't appear when two or more elements are selected. When that happens you should see the Activate Dimensions button appear on the Options Bar.
Temporary dimensions also don't respond to families that host nested shared families. That's because these families are also regarded by Revit, under the hood, as a selection of two or more elements.